Carbon-coated lithium titanium spinel

ABSTRACT

A carbon-containing lithium titanium oxide containing spherical particle aggregate with a diameter of 1-80 μm, consisting of lithium titanium oxide primary particles coated with carbon. Also, a method for the production of such a carbon-containing lithium titanium oxide as well as an electrode containing such a carbon-containing lithium titanium oxide as active material as well as a lithium-ion secondary battery containing an above-described electrode.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a National Phase application of PCT application numberPCT/EP2009/007196, filed Oct. 7, 2009, which claims priority benefit ofGerman application number DE 10 2008 050 692.3, filed Oct. 7, 2008, thecontent of such applications being incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to carbon-coated lithium titanateLi₄Ti₅O₁₂ as well as a method for its production.

BACKGROUND OF THE INVENTION

The use of lithium titanate Li₄Ti₅O₁₂, or lithium titanium spinel forshort, in particular as substitute for graphite as anode material inrechargeable lithium-ion batteries was proposed some time ago.

A current overview of anode materials in such batteries can be founde.g. in: Bruce et al., Angew. Chem. Int. Ed. 2008, 47, 2930-2946.

The advantages of Li₄Ti₅O₁₂ compared with graphite are in particular itsbetter cycle stability, its better thermal load capacity as well as thehigher operational reliability. Li₄Ti₅O₁₂ has a relatively constantpotential difference of 1.55 V compared with lithium and achievesseveral 1000 charge and discharge cycles with a loss of capacity of<20%.

Thus lithium titanate has a clearly more positive potential thangraphite which has previously customarily been used as anode inrechargeable lithium-ion batteries.

However, the higher potential also results in a lower voltagedifference. Together with a reduced capacity of 175 mAh/g compared with372 mAh/g (theoretical value) of graphite, this leads to a clearly lowerenergy density compared with lithium-ion batteries with graphite anodes.

However, Li₄Ti₅O₁₂ has a long life and is non-toxic and is thereforealso not to be classified as posing a threat to the environment.

Recently, LiFePO₄ has been used as cathode material in lithium-ionbatteries, with the result that a voltage difference of 2 V can beachieved in a combination of Li₄Ti₅O₁₂ and LiFePO₄.

Various aspects of the production of lithium titanate Li₄Ti₅O₁₂ aredescribed in detail. Usually, Li₄Ti₅O₁₂ is obtained by means of asolid-state reaction between a titanium compound, typically TiO₂, and alithium compound, typically Li₂CO₃, at high temperatures of over 750°C., as described in U.S. Pat. No. 5,545,468 or EP 1 057 783 A1.

Sol-gel methods for the production of Li₄Ti₅O₁₂ are also described (DE103 19 464 A1). Furthermore, production methods by means of flame spraypyrolysis are proposed (Ernst, F. O. et al. Materials Chemistry andPhysics 2007, 101 (2-3, pp. 372-378) as well as so-called “hydrothermalmethods” in anhydrous media (Kalbac, M. et al., Journal of Solid StateElectrochemistry 2003, 8 (1) pp. 2-6).

Since the lithium titanate as electrode is typically compressed to anelectrode with carbon, in particular graphite or carbon black, EP 1 796189 A2 proposes providing complex lithium transition metal oxides exsitu, i.e. after their complete synthesis with a carbon-containingcoating. A disadvantage with this method, however, is the large particlesize of the contained product, in particular the secondary particlesize. Moreover, the carbon coating in this method is located on thesecondary and not the primary particles, which leads to poorelectrochemical properties, in particular as regards its capacitybehaviour.

There was therefore a need to provide a further lithium titanium oxide,in particular a lithium titanate Li₄Ti₅O₁₂, which has particularly smallparticles and improved electrochemical properties.

According to the invention, this object is achieved by acarbon-containing lithium titanium oxide containing spherical(secondary) particle aggregates with a diameter of 1-80 μm consisting oflithium titanium oxide primary particles coated with carbon.

The German terms “Partikel” and “Teilchen” here are used synonymously tomean particle.

In the following, by lithium titanium oxide is meant a lithium titanatewhich according to the invention includes all lithium titanium spinelsof the type Li_(1+x)Ti_(2−x)O₄ with 0≦x≦1/3 of the spatial group Fd3mand generally also all mixed lithium titanium oxides of the genericformula Li_(x)Ti_(y)O (0<x, y<1).

The carbon-coated lithium titanium oxide according to the inventionconsists, as mentioned, of secondary particles which are formed ofprimary particles coated with carbon. The secondary particles arespherical in shape.

The result of the particle size according to the invention of thesecondary particles is that the current density in an electrode thatcontains the carbon-coated lithium titanium oxide material according tothe invention is particularly high and it has a high cycle stabilitycompared with the materials of the state of the art, in particular EP1796 189 A2.

Surprisingly, it was also found that the carbon-containing lithiumtitanium oxide according to the invention has a BET surface area(measured in accordance with DIN 66134) of 1-10 m²/g, preferably <10m²/g, still more preferably <8 m²/g and quite particularly preferably <5m²/g. In a quite particularly preferred embodiment, typical values liein the range of from 3-5 m²/g.

The primary particles coated with carbon typically have a size of <1 μm.It is important according to the invention that the primary particlesare small and at least partially coated with carbon, with the resultthat the current-carrying capacity and the cycle stability of anelectrode containing the lithium titanium oxide according to theinvention are particularly high compared with non-carbon-coatedmaterials or materials which are not homogeneously coated or comparedwith materials in which only the secondary particles are coated.

In preferred embodiments of the present invention, the carbon content ofthe lithium titanium oxide according to the invention is 0.05 to 2wt.-%, in quite particularly preferred embodiments 0.05 to 0.5 wt.-%.

Surprisingly, it was found that relatively low carbon contents, i.e.thus a relatively thin carbon coating of the primary particles, aresufficient to bring about the above-mentioned advantageous effects inelectrodes which contain the material according to the invention.

Of the lithium titanium oxides, Li₄Ti₅O₁₂ is preferred because it isparticularly well-suited as electrode material.

DESCRIPTION OF THE INVENTION

The object of the present invention is further achieved by a method forthe production of carbon-containing lithium titanium oxide comprisingthe steps of

-   (a) mixing a lithium salt, a titanium oxide and a carbon-containing    compound in a solvent;-   (b) drying the mixture from step a);-   (c) calcining the dried mixture

Depending on the ratios of the lithium salt to titanium oxide, thelithium titanium spinels Li_(1−x)Ti_(2−x)O₄ as already described aboveof the spatial group Fd3m or the mixed lithium titanium oxides of thegeneric formula Li_(x)Ti_(y)O can be obtained.

The final carbon content of the lithium titanium oxide according to theinvention can also be set during the mixing.

The term “solvent” is here defined such that at least one constituent ofthe starting substances is at least partially soluble in the solvent,i.e. has a solubility product L of at least 0.5. The solvent ispreferably water. In quite particularly preferred embodiments, oneconstituent of the starting substances is readily soluble in water, i.e.it has a solubility product L of at least 10.

Particularly preferably, the atomic ratio of Li to Ti is 4:5, with theresult that in particular phase-pure Li₄Ti₅O₁₂ with a carbon coating canbe obtained. By phase-pure is meant here that, within the limits of theusual measurement accuracy, no TiO₂ can be detected in the rutile phaseby means of XRD measurements.

Preferably, the lithium salt for carrying out the method according tothe invention is selected from the group consisting of LiOH, LiNO₃,Li₂CO₃, Li₂O, LiHCO₃, and lithium acetate, since an aqueous solution towhich the other starting compounds can be added can be producedparticularly easily from these starting compounds.

Preferably, TiO₂ in anatase form or in amorphous form is used, whichadvantageously does not change into rutile as a result of the methodaccording to the invention.

The carbon-containing compounds which are suitable for carrying out themethod according to the invention are selected for example fromhydrocarbons, such as for example polycyclic aromatics and theircompounds, perylene and its compounds, polymers and copolymers, such asfor example polyolefins, polypropylene copolymers in powder form,styrene-polybutadiene block copolymers, sugars and their derivatives.Particularly preferred polymers are polyolefins, polybutadienes,polyvinyl alcohol, condensation products from phenol, polymers derivedfrom furfuryl, styrene derivatives, divinylbenzene derivatives, naphtholperylene, acrylonitrile and vinyl acetate, gelatin, cellulose, starchand their esters and ethers and mixtures thereof.

The choice of sugars has proved to be quite particularly preferred forcarrying out the method according to the invention, since these dissolveparticularly well in water. Of the sugars, lactose, sucrose andsaccharose are quite particularly preferred, lactose being the mostpreferred.

The drying step b) typically takes place in the form of so-called spraydrying, in which the obtained mixture is finely sprayed through a nozzleand precipitates in the form of a pre-product. However, any other methodin which the starting compounds are homogeneously mixed and thenintroduced into a gas stream for drying can also be used. In addition tospray drying, these methods are for example fluid-bed drying, rollinggranulation and drying or freeze-drying alone or in combination. Spraydrying is quite particularly preferred and typically takes place in atemperature gradient of from 90-300° C.

After obtaining the dried product of the aqueous mixture from step a),which advantageously also avoids the solvent problems of other methodsof the state of the art, the obtained spray-dried pre-product iscalcined at a temperature of from 700 to 1000° C., preferably under aprotective atmosphere, in order to avoid possible secondary reactionsduring the calcining which could lead to undesired results, such as e.g.the oxidation of the carbon coating. Suitable protective gases are e.g.nitrogen, argon, etc. or mixtures thereof.

The present invention also relates to a lithium titanium oxideobtainable by the method according to the invention which ischaracterized by a particularly small BET surface area and a smallparticle size of the primary particles as well as of the secondaryparticles formed from the primary particles, as has already beendescribed above.

The problem of the present invention is further solved by an electrodewhich contains the carbon-coated lithium titanium oxide according to theinvention. Preferably, the electrode is an anode. In particular, it wasfound here that such an electrode has a capacity ratio between 1C and 4Cof >85% and a discharge capacity of at least 165 mAh/g at C/10 in alithium-ion secondary battery.

BRIEF DESCRIPTION OF THE DRAWING

The present invention is described in more detail below with referenceto the embodiment examples as well as the figures which are not,however, to be considered limiting.

There are shown in:

FIG. 1 an SEM micrograph of carbon-coated Li₄Ti₅O₁₂ according to theinvention;

FIG. 2 the diagram showing the charge and discharge capacity of anelectrode containing an (in-situ) carbon-coated lithium titanateaccording to the invention;

FIG. 3 the charge and discharge capacity of ex-situ coated lithiumtitanate as comparison;

FIG. 4 an SEM micrograph of a subsequently (ex-situ) carbon-coatedLi₄Ti₅O₁₂;

FIG. 5 an SEM micrograph of uncoated Li₄Ti₅O₁₂;

FIG. 6 the charge and discharge capacity of the uncoated Li₄Ti₅O₁₂; thecurrent was the same during charging and during discharging.

EMBODIMENT EXAMPLES

1. General

LiOH.H₂O as well as TiO₂ in anatase form are used below as startingproducts. The water content in the case of commercially availableLiOH.H₂O (from Merck) varies from batch to batch and was determinedprior to synthesis.

A suspension of LiOH/TiO₂/lactose was produced at 30-35° C., by firstdissolving LiOH.H₂O in water and then adding TiO₂ in anatase form aswell as lactose while stirring:

Example 1

Production of the Lithium Titanate According to the Invention(Li₄Ti₅O₁₂)

9.2 kg LiOH.H₂O was dissolved in 45 l water and then 20.8 kg TiO₂ wasadded. Different quantities of lactose were then added. The quantity oflactose was varied further, batches with 30 g lactose/kg LiOH+TiO₂, 60 glactose/kg LiOH+TiO₂, 90 g lactose/kg LiOH+TiO₂ were being run in orderto vary the quantity of carbon in the lithium titanate according to theinvention.

It was surprisingly found that the lactose had the effect of reducingthe viscosity of the original suspension, with the result that 25% lesswater had to be used for the production of a corresponding suspensionthan in the case without the addition of the lactose. The mixture wasthen spray dried in a Nubilosa spray dryer at a starting temperature ofapprox. 300° C. and an end temperature of 100° C.

First, porous spherical aggregates of the order of several micrometresformed.

Then, the product obtained in this way was calcined at 800° C. for onehour under nitrogen atmosphere. Large (1-80-μm) aggregates of aggregatedprimary particles (particle size of the primary particles <1 μm) wereobtained.

FIG. 1 shows the carbon-coated lithium titanate according to theinvention with 0.2 wt.-% total carbon content (60 g lactose/kgLiOH+TiO₂), while FIG. 5 shows an uncoated lithium titanate alsoobtained by means of spray drying. The carbon-containing compound in thestarting products of the method according to the invention acts assintering incubator and leads to clearly smaller particles.

Comparison Example

Uncoated lithium titanate was produced according to the method fromExample 1, i.e. without addition of lactose.

The thus-obtained and calcined lithium titanate was then impregnatedwith lactose solution for 3 h and heated for 3 h at 750° C. (cf. EP 1796 198 A2). An SEM micrograph of the product is represented in FIG. 4and shows, compared with the product according to the inventionaccording to FIG. 1, clearly coarser particles which likewise consist,not of primary particles with a size <1 μm, but of larger primaryparticles sintered together. In addition, the secondary particles of thecomparison example have a “smeared” coating. The carbon content waslikewise approx. 0.2 wt.-%.

Charge/discharge cycles were then carried out with the materialaccording to the invention as well as with the material of thecomparison examples, i.e. with the subsequently coated lithium titanate(according to EP 1 796 198 A2) as well as with the uncoated lithiumtitanate which were both obtained by means of the same method.

The anode consisted in each case of 85% active material, 10% Super Pcarbon black and 5% PVDF 21256 binder. The measurements took place withthe material according to the invention or comparison materials as anodein a half cell compared with metal lithium. The active mass content ofthe electrode was 2.2 mg/cm². The range covered in the cycles was1.0-2.0 volts. FIG. 2 shows charge/discharge curves of carbon-coatedlithium titanate according to the invention, wherein the capacity ratiobetween 1C and 4C was 87.5%; the current was the same during chargingand during discharging.

Compared with the uncoated lithium titanate, a clear stability is to beobserved which according to FIG. 6 has a corresponding capacitybehaviour of 82%.

Likewise, compared with an ex-situ coated lithium titanate (FIG. 3), thematerial according to the invention, in which only 75% of the capacitywas measured at 4C, is better. The current was the same during chargingand during discharging.

The results thus show that the coated in-situ carbon-coated lithiumtitanate according to the invention has major advantages as regards itscapacity ratio compared with a subsequently applied carbon coating oruncoated lithium titanate.

1. A carbon-containing lithium titanium oxide-containing sphericalparticle aggregates with a diameter of from 1 to 80 μm which consists oflithium titanium oxide primary particles coated with carbon.
 2. Theaggregate of claim 1 with a BET surface area in the range of from 1-10m²/g.
 3. The aggregate of claim 2 with a primary particle size of <1 μm.4. The aggregate of claim 3 with a carbon content of from 0.05 to 2wt.-%.
 5. The aggregate of claim 4 with a carbon content of from 0.05 to0.5 wt.-%.
 6. The aggregate of claim 1, wherein the lithium titaniumoxide is Li₄Ti₅O₁₂.
 7. A method for the production of carbon-containinglithium titanium oxide according to claim 1, comprising the steps of:(a) mixing a lithium salt, a titanium oxide and a carbon-containingcompound in a solvent; (b) drying the mixture from step a); (c)calcining the dried mixture.
 8. The method of claim 7, wherein thesolvent is water.
 9. The method of claim 8, having an atomic ratio Li/Tiof 4:5.
 10. The method of claim 8, wherein the lithium salt is selectedfrom the group consisting of LiOH, Li₂O, LiNO₃, LiHCO₃, and LiCH₃COO.11. The method of claim 9, wherein the TiO₂ in anatase form or inamorphous form is used.
 12. The method of claim 11, wherein thecarbon-containing compound is selected from the group consisting ofhydrocarbons and their derivatives, carbohydrates and their derivatives,and polymers.
 13. The method of claim 12, wherein the carbon-containingcompound is selected from sugars of the group consisting of lactose,sucrose and saccharose.
 14. The method of claim 13, wherein the dryingis carried out as spray drying.
 15. The method of claim 14, wherein thespray drying is carried out at a temperature gradient of from 90-350° C.16. The method of claim 7, wherein the calcining is carried out at atemperature of from 700 to 1000° C. under protective atmosphere.
 17. Alithium titanium oxide obtained by the method of claim
 7. 18. Anelectrode comprising the carbon-containing lithium titanium oxideaggregate of claim
 1. 19. A lithium-ion secondary battery comprising theelectrode of claim
 18. 20. The lithium-ion Lithium-ion secondary batteryaccording to claim 19 with a charge/discharge capacity at C/10 of >165mAh/g.